Shrink-wrap packaging and apparatus and methods for producing the same and particularly to shrink-wrap packaging and apparatus and methods for feeding and cutting a shrink-wrap film utilized in producing the shrink-wrap packaging are shown and described.
In the packaging industry, the need for faster production lines running at maximum efficiency is at an all time high. Manufacturers are operating 24 hours per day, 7 days per week just to maintain consumer product demands. Obviously, the machinery produced to operate on these packaging lines need to keep pace, if not, exceed the expectations and demands of the end users. Also, more manufacturers are turning to shrink-wrap packaging due to the lower cost of plastic as compared to corrugated boxes along with the ability to use full color graphics to produce aesthetically pleasing packages in an attempt to differentiate themselves from competitors.
In shrink-wrapping, a single sheet of shrink-wrap film is wrapped around the product and into a tubular form. The overlapping lateral edges are located beneath the product and are sealed or otherwise joined together. During shrinking in a heat tunnel, the longitudinal edges of the shrink-wrap film collapse against the ends of the product creating bull's eye-type openings.
U.S. Pat. Nos. 5,771,662 and 7,032,360 represented a major advance in the field of feeding, perforating and cutting a shrink-wrap film. Specifically, methods and apparatus are disclosed in U.S. Pat. Nos. 5,771,662 and 7,032,360 for forming a single sheet of film from a web of film and connected to the web of film by tie strips to maintain tension on the web of film after cutting, with the tie strips later being broken to separate the single sheet of film from the web of film.
The speeds of shrink-wrap packaging machinery were limited by film cutter technology that could accurately and consistently cut the film at cycle rates high enough to keep pace with production. No devices hold or grasp the film while cutting so manufacturers encounter many challenges when cutting film at high cycle rates. These challenges include twisting of shafts when run at high speeds, torsional shaft oscillations causing the cutter knife to miss film cuts, the time required for the shaft material to adequately dampen out any torsional oscillations (vibrations) before the next cut event occurs, a “jump roping” effect where even a perfectly balanced shaft begins to deflect laterally when turned at a particular resonant rpm causing the knife to hit upon the support deck because the shaft is jump roping, and reduced bearing life.
Particularly, conventional cutter shafts had deficiencies which limited its cycle rate to approximately 105 cuts per minute. Unwieldy shaft twist resulted when attempting to run at speeds over 105 cuts per minute. Frequency and magnitude of the torsional shaft oscillations made high performance servo loop control difficult, if not impossible. Thus, servo gains could not be set high enough to achieve the quality of motion precision for high speed applications. When servo gains are turned down to accommodate inferior shaft designs, the shaft might not cut where desired because the servo cannot adequately reproduce the commanded position. Furthermore, the knife can demonstrate trajectory over shoot and subsequent oscillations as it tries to bring the knife to rest at the stop location. If the overshoot is excessive, it causes the film trajectory to deviate from the desired film path. The film trajectory is likely deviated due to air flow disturbance caused by the knife overshoot. The time required for the shaft material to adequately dampen out any torsioinal oscillations before the next cut event occurs was too long. If the shaft still exhibited torsion motion, this can adversely affect the next cut performance due to the existence of this unwanted energy which must be overcome. Shaft rpm was limited, because the shaft would demonstrate the phenomenon known as “shaft rotational critical frequency”. This phenomenon is where the shaft begins to deflect laterally when turned at a particular resonant rpm, even if the shaft is perfectly balanced. The lateral deflection of the shaft causes what looks like a jump rope effect. The jump rope effect is undesirable as it causes the periphery of the knife to deviate from the commanded trajectory, e.g. the knife would hit upon the support deck.
Thus, there is a continuing need to improve the function, to reduce maintenance, to provide enhanced adjustability and to reduce the number and complexity of components in an apparatus for producing shrink-wrap packaging and to overcome the challenges in the field of film cutting and handling.